I'm going to bet that they'll go for something more traditional or even more bizarre after seeing the Warlock tier.

Either way there needs to be some flavor, modeling the gear based on the raid seems to just produce very bland sets for priests. They try to appeal to both shadow and healing priests, but with the former constraint in place they just end up uninspired.

If they play safe, it might be something based off the Spirit Healer. It would make sense, something along the line of Deathwing raid so hard you'll be wiping forever, so dead priest aka spirit healer.

Well, so far it looks like they are going for some big epic themes with the sets, so I doubt we'll see any more random elemental themes. Though I do think that warrior tier is kinda way better quality than lock one (just make lock one not so bright and happy, make gloves that do not look like a part of princess dress and make shoulders that look cloth, not leather).

Either way, it's hard to propose anything, since we had no real presence of Church of the Holy Light in this expansion and no bosses at least remotely connected to the Light. If it was LK I would suggest Argent Crusade and tournament theme, but in cata it was completely ignored. I think this is why us and some other classes(hunters, rogues, DKs) suffer from rather random sets, which don't make much sense.

Only thing even remotely close to priest theme in this expansion will be cleansing flame and *maybe* red dragon flight, so we might see another red themed set.